The present invention relates to a method for preventing polymer scale deposition on reactor walls in the polymerization of an ethylenically unsaturated monomer in an aqueous medium.
Ethylenically unsaturated monomers are polymerized in several different ways including suspension or emulsion polymerization in an aqueous medium, solution polymerization, gas-phase polymerization and bulk polymerization according to the kind of the monomer and the desired properties of the polymer. Among the above named methods of polymerization, several monomers are almost exclusively polymerized in an aqueous medium. Examples of such monomers are vinyl chloride, vinylidene chloride and styrene.
Not only the suspension and emulsion polymerizations but also any other polymerization processes suffer from the problem of polymer scale deposition on the walls of the polymerization reactor and other surfaces of the reactor coming into contact with the monomer as one of the most difficult problems in the industrial production of polymer products.
The polymer scale deposition on the reactor walls must be prevented as completely as possible because not only of the decrease in the yield of the polymer product and lowering of the cooling capacity of the reactor but also of the degraded quality of the polymer product due to the intermingled fragments of the polymer scale in the product coming off the reactor walls. Further, the works of removing polymer scale require much time and labor with consequently decreased productivity and increased production costs of the polymer products. What is worse, unreacted monomers are included in a large amount in the polymer scale and the workers undertaking the removal of the scale are exposed to the vapor of the monomer which is very dangerous due to its toxicity to the human health.
Various attempts have been proposed hitherto for reducing the amount of polymer scale deposition on the reactor walls. For example, polymer scale deposition on the reactor walls in the aqueous suspension polymerization of vinyl chloride can be reduced by coating the reactor walls, prior to polymerizaion, with a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds and the like or an organic dye (see, for example, U.S. Pat. No. 3,669,946). This method is considerably effective in preventing polymer scale deposition but the problem in the method is the use of an organic solvent to dissolve the organic compound or the dye which is not or sparingly soluble in water in order to obtain a coating solution. Needless to say, the use of an organic solvent is undesirable due to the toxicity to the human body as well as the danger of fire or explosion. When the organic solvent is replaced with water instead of the organic solvent, if it is ever possible, the expected effect of polymer scale prevention is largely reduced to an impractically low level.
Further, the effectiveness of the above described method is considerably durable when the polymerization of vinyl chloride is carried out as suspension polymerization but, when the polymerization medium contains an emulsifying agent such as in the emulsion polymerization or in the combined use of an emulsifying agent with a suspending agent, effectiveness of the coating method is limited and not durable.
Furthermore, there has been proposed no promising method for preventing polymer scale deposition in the polymerization of styrene or copolymerization of styrene and butadiene or styrene, acrylonitrile and butadiene, for example, in an aqueous medium. Therefore, these polymerization procedures are conducted in a glass-lined polymerization reactor in contrast to stainless steel-made reactors widely used for the polymerization of vinyl chloride in an aqueous medium. Several of the problems in a glass-lined polymerization reactor are that the heat transfer coefficient of the glass-lined reactor walls is much lower than the walls of stainless steel reactors bringing about difficulties in effectively controlling the temperature, that glass-lined walls are susceptible to mechanical damage or fracture resulting in shorter life of the vessel and that a large-size reactor is hardly obtained due to the difficulty in fabrication despite the advantage that the glass-lined surface is less susceptible to the polymer scale deposition than the stainless steel surfaces.
One of the reasons for the relative ineffectiveness of aqueous coating solutions used for the purpose on stainless steel surfaces is that, due to the remarkably large surface tension of the aqueous solution, the coating solution cannot spread uniformly over the surface since metal surfaces are more or less repellent to aqueous solutions. Addition of a surface active agent to the aqueous coating solution is effective in improving the spreading of the solution on a metal surface but surface active agents generally increase the polymer scale deposition contrary to the object of coating.
Accordingly, there has been eagerly desired to develop a novel method for the prevention of polymer scale deposition on the reactor walls which is effective not only in the suspension polymerization of vinyl chloride but also in the polymerization of different types of vinyl chloride as well as in the polymerization or copolymerization of monomer or monomers other than vinyl chloride in an aqueous medium and in which no organic solvent is used for the preparation of the coating solution to be applied on to the reactor walls.